Thursday, April 14, 2016

ECDC Influenza Virus Characterization Report - March 2016

Credit NIAID

#11,268

Once a month during the winter, and a little less regularly during the summer and fall, the ECDC releases a report on the influenza viruses collected and submitted by their member nations to the Crick Worldwide Influenza Centre(WIC).

Seasonal flu viruses (H1N1, H3N2, and Victoria & Yamagata B Viruses) continually evolve; each can have multiple strains co-circulating at the same time.

Some of these strains are more `biologically fit', and are therefore more successful, than others. As populations gain immunity against the dominant strain, new strains evolve to replace it.

Making even small or incremental changes in the flu viruses circulating around the world important to keep track of.

While many of the samples submitted since the start of 2016 have yet to be analyzed, the ECDC's March report provides us with detailed snapshot of the flu viruses circulating in Europe into early January.

Follow the link to download the full (and highly technical) PDF report. When you return, I'll have a bit more.

​From week 40/2015, the start of weekly reporting on influenza activity in the WHO European Region, to week 13/2016, over 120 000 influenza detections across the Region have been reported. Influenza type A viruses are prevailing over type B but, unlike the situation in the 2014–15 season, A(H1N1)pdm09 viruses are prevailing over A(H3N2) and the proportion of B/Victoria-lineage detections has risen substantially, representing ~94% of those assigned to a lineage.

To date, 24 EU/EEA countries have shared 472 influenza-positive specimens with the Francis Crick Institute, London, for detailed characterisation: one additional country and 48 specimens since the February 2016 report. Since the latter report, 67 viruses have been characterised antigenically and genetic analyses are ongoing.

The 35 A(H1N1)pdm09 viruses characterised antigenically were similar to the vaccine virus A/California/7/2009. Worldwide, new genetic sub-clusters of viruses within the 6B clade have emerged, with two being designated as subclades: 6B.1 defined by HA1 amino acid substitutions S162N and I216T and 6B.2 defined by HA1 amino acid substitutions V152T and V173I. Of the 178 viruses characterised genetically for the 2015–16 season, 24 (13%) were clade 6B, 147 (83%) were subclade 6B.1 and seven (4%) were subclade 6B.2.

The six A(H3N2) test viruses characterised by haemagglutination inhibition (HI) assay were poorly recognised by reference antiserum raised against egg-propagated A/Switzerland/9715293/2013, the vaccine virus recommended for use in the 2015–16 northern hemisphere influenza season, despite at least three of the test viruses falling in the same subclade (3C.3a) as the vaccine virus. The test viruses were recognised somewhat better by antisera raised against egg-propagated A/Hong Kong/4801/2014, the virus recommended for use in 2016 southern hemisphere and 2016–17 northern hemisphere influenza vaccines. Of 55 A(H3N2) viruses characterised genetically for the 2015–16 season: one (2%) was clade 3C.3, 32 (58%) were subclade 3C.2a and 22 (40%) were subclade 3C.3a.

The 23 B/Victoria-lineage viruses were antigenically similar to tissue culture-propagated surrogates of B/Brisbane/60/2008. All 56 viruses characterised genetically for the 2015–16 season fell in genetic clade 1A as do recently collected viruses worldwide.

Three B/Yamagata viruses have been characterised since the previous report; all reacted well with post-infection ferret antiserum raised against egg-propagated B/Phuket/3073/2013, the recommended vaccine virus for the northern hemisphere 2015–16 influenza season and for quadrivalent vaccines in the 2016 southern hemisphere and 2016–17 northern hemisphere seasons. All 10 viruses characterised genetically for the 2015–16 season fell in genetic clade 3.

Despite growing diversity in the H1N1 flu virus, for now indications
are that the vast majority of H1N1 viruses in circulation continue to
be a good antigenic match the 2009 H1N1 vaccine strain.

And the growing availability of the Quadrivalent Flu vaccine, containing both Victoria and Yamagata lineage B strains (see Two B's Or Not Two B's? That Is the Question) will take some of the pressure off predicting which lineage will dominate in the coming year.

A final note, though. In January and February Russia reported very heavy influenza
activity, and theirEpidemic Surveillance centre reported a small number of H1N1 virus samples showing reduced titers against the current vaccine strain.

So far, `low reactor' H1N1 viruses have not turned up in the ECDC reporting this year, but we'll keep an eye on the next couple of reports to see if that changes.